Geochemical and petrographic characteristics and magmatic evolution of the Los Azufres Volcanic Field, Michoacán, during the Pleistocene

Abstract

Los Azufres Volcanic Field hosts the second most important geothermal field in Mexico. Volcanic activity in this field began some 1.57 Ma and lasted until < 26 ka, with the emission of different volcanic products with compositions varying from basaltic andesite to rhyolite. The volcanic products generated by this chemically heterogeneous activity are lava plateaus, domes, and stratovolcanoes, many of them with pyroclastic deposits associated. All these rocks were grouped into four different units: Basaltic-Andesite, Andesitic, Dacitic and Rhyolitic. The rhyolitic products are the most abundant with ages ranging from 1.5 Ma to <26ka, whereas the mafic units vary from 1.5 to 0.33 Ma. The mineralogy observed in the different units is consistent with their chemical composition, the Basaltic-Andesite Unit contains ol+plg+opx+cpx and oxides, while the Andesitic Unit contains plg+opx+oxides, the Dacitic Unit contains plg+opx+cpx+anf+bt and oxides, as well as traces of quartz and zircon, whereas the Rhyolitic Unit contains qz+sanidine+plg+bt+anf and oxides. In general, the major element concentrations of all the analyzed samples display clear evolutive trends, which suggest a genetic relationship, with a break in slope configured by the geochemical data at approximately 64 wt. % SiO₂ for most elements that can be related to the fractionation of mafic mineral phases. Regarding trace elements, there is an enrichment in incompatible and light rare earth elements, with pronounced positive anomalies in Pb, U and K and negative in Nb, Ta, and Ti, in addition to a variable negative Eu anomaly depending on the chemical composition. According to our petrogenetic model generated in this work, the main evolutive mechanism responsible for the magmatic diversity in the CVLA was the assimilation of crustal rocks, with composition like that of La Huacana intrusive rocks, coupled with fractional crystallization (AFC). The results show an assimilated mass/crystallized mass ratio of 0.5, crystallizing 54 % of the magma, assuming the fractionation of 5 % olivine, 10 % clinopyroxene, 10 % orthopyroxene, 20 % plagioclase, 5 % amphibole and 5 % magnetite. To a lesser extent, assimilation without fractional crystallization also played a role in the magmatic evolution, although the model does not explain the generation of most rhyolitic magmas in the area, which were most likely derived by a different mechanism not discussed here.